The subject matter of this application relates to circuit and method for distinguishing between an orthogonally frequency division multiplexed (OFDM) signal and a radar signal.
In several digital modulation techniques, a group of consecutive data bits in an input data stream is represented by a symbol D. Different combinations of data bits are represented by different symbols. For example, in the case of the group being composed of two bits, there are four possible combinations and accordingly there are four different symbols. One common type of digital modulation employing four symbols is 4-level phase shift keying (QPSK), in which the four different symbols induce four equiangularly spaced values of phase displacement in the bandpass representation.
In the complex baseband representation, each symbol D is conventionally represented by a doublet (I, Q). The I and Q components of the symbol are applied to respective ports of a quadrature modulator that also receives a carrier signal and outputs a signal that is modulated in frequency and/or phase and/or amplitude in accordance with the values of the I and Q components.
Orthogonal frequency division multiplexing (OFDM) is a digital modulation technique in which an input data stream is decomposed into several subsidiary streams, each subsidiary stream is represented by a sequence of symbols, and the several sequences of symbols are used to modulate respective carriers of constant frequency. The modulated carriers are summed to produce a transmission signal, which is supplied to a transmitter antenna for transmission to a receiver antenna. Receivers equal in number to the carriers and tuned to the carriers respectively receive and detect the sequences of symbols. Each sequence of symbols is then used to recover the corresponding subsidiary data stream, and the subsidiary data streams are combined in order to recreate the original data stream. The carriers are sufficiently spaced in frequency that they are orthogonal, i.e. each receiver sees only its own carrier.
FIG. 1 illustrates in very simplified form a transmitter and receiver for use with OFDM. Referring to FIG. 1, the transmitter includes a fixed-frequency local oscillator 10 generating a carrier at constant frequency f and N (as many as 100) subcarrier channels 16, each of which includes a subcarrier oscillator 18 generating a subcarrier at a selected constant frequency F. The different oscillators 181-18N operate at different respective, mutually orthogonal, frequencies F1-FN. Each subcarrier channel 16 also includes a quadrature phase shift key (QPSK) modulator 20 which modulates the subcarrier in phase based on the value of a two bit data word D to provide a QPSK modulated output signal and thereby encodes the subcarrier with the data word D.
The output signals of the subcarrier channels 16 are summed and the resulting composite subcarrier signal is mixed with the output signal of the local oscillator 10 to produce a transmission signal, which is supplied to a transmitter antenna 22 for transmission to a receiver antenna 24. The transmission signal includes signal components at frequencies (f+F1), (f+F2), . . . (f+FN). Thus, the transmission signal occupies a block of transmission frequencies from (f+F1) to (f+FN). The subcarrier frequencies F1-FN are chosen so that the transmission frequencies do not overlap and are sufficiently spaced to avoid interference.
A receiver that is connected to the receiver antenna 24 includes a receiver local oscillator 26 operating at the same constant frequency f as the transmitter oscillator 10. The receiver LO signal is mixed with the receiver antenna signal and provides an output signal that contains frequency components at the N subcarrier frequencies F1-FN respectively and is supplied to N receiver subcarrier channels 28, tuned to the subcarrier frequencies F1-FN respectively. Each receiver subcarrier channel includes a QPSK demodulator 30 that recovers the data words D that were encoded by the corresponding QPSK modulator 20.
It will be appreciated that although FIG. 1 illustrates separate functional blocks for the several functions that are performed by the transmitter and receiver, in practice many of the functions may be combined in a single digital signal processor.
One application of OFDM is in implementation of the standard commonly known as IEEE 802.11a-1999. IEEE 802.11a-1999, or simply 802.11a, prescribes the physical layer for a wireless local area network (WLAN) that operates in the 5 GHz frequency band (the middle range) of the Unlicensed National Information Infrastructure (U-NII) utilizing an OFDM-based air interface. In a typical implementation, the 802.11a signal occupies a bandwidth of at least 20 MHz. Extensions of the 802.11a standard, such as 802.11n and 802.11ac, allow multiple 20 MHz channels to be bonded together up to a bandwidth of 160 MHz.
Regulations effective in the United States require that a transmitter that operates in the middle range of the U-NII radio band should constantly monitor its radio channel for radar signals and move to another frequency channel when a radar signal has been detected. Some radar systems use a modulated single carrier waveform having a constant frequency whereas other systems use a signal that periodically sweeps through a range of frequencies. A radar detector must be able to detect both a single tone radar signal and a sweeping, or chirping, radar signal.
A conventional radar detector that may be used in an 802.11a wireless device may process incoming signals to identify pulses and classify the pulses based on the average logarithm of the pulse width and pulse repetition rate. A logarithmic PW/PRR mask of known radar types is applied to the average signal parameters and a radar detection is reported when a match is found. This approach is not able to differentiate between a radar signal and a signal having a similar pulse width and pulse repetition rate to a known radar type and may therefore lead to false radar detections.